Although the etiology of multiple sclerosis (MS) has not been established, it is widely believed that immunological mechanisms are involved. Experimental autoimmune encephalomyelitis (EAE) is the primary animal model used in the study of MS. EAE can be induced in animals by immunization with either CNS tissue, myelin, or myelin components. In MS and EAE, there is an increased CNS infiltration of T-lymphocytes. These infiltrating T-cells secrete the cytokine interferon-gamma (IFN-gamma), which is not normally present within, the CNS. We hypothesize that IFN-gamma is a key mediator of demyelination in MS -and EAE. The goal of this proposal is to examine the effects of IFN-gamma on the myelination process and the myelin sheath using several in vivo models. The effects of IFN-gamma on the myelination process will be characterized in detail in transgenic mice in which the expression of IFN-gamma has been targeted to the CNS. These transgenic (MBP/IFN-gamma) animals are severely hypomyelinated, strongly suggesting that the presence of IFN-gamma is detrimental to the myelination process. We will also examine the effects of IFN- gamma on an already well-myelinated CNS using microosmotic pumps to deliver this cytokine to the adult CNS. IFN-gamma has been suggested to have an indirect effect in demyelinating disorders through the activation of macrophages/microglial cells, which in turn produce substances (tumor necrosis factor and nitric oxide) that are thought to be cytotoxic to oligodendrocytes. We speculate that IFN-gamma may also have a direct effect on oligodendrocytes. In an effort to begin to understand the cellular site of action of IFN-gamma in the CNS, we will examine the effects of this cytokine in transgenic mice in which macrophages, and presumably microglia, are unresponsive to IFN-gamma. Moreover, various parameters of EAE will be examined in SJL mice in which the IFN-gamma gene has been genetically inactivated. Together, we believe that these studies will significantly further our understanding of the potential effects and site of action of IFN-gamma in immune-mediated demyelinating disorders of the CNS. Such information is critical for the rational design of therapeutic strategies for these disorders.